Process for operating a coal-based fluidized bed combustor and fluidized bed combustor

ABSTRACT

Some part (a) of the bed material is withdrawn continuously (9e, 10) from the lower region of the fluidized bed (20), cooled (11), screened (12, 13) on a belt (a&#39;) to obtain a particle size specific to the fluidized bed, and then recycled to the fluidized bed (15, 18).

BACKGROUND OF THE INVENTION

The invention relates to a process for operating a fluidized bedcombustor on coal basis in which comminuted coal is fed into a fluidizedbed and burned with the bedding material. The invention furthermorerelates to a fluidized bed combustor with a conveyor to feed the coaland with a nozzle bottom on which a fluidized bed consisting of beddingmaterial is positioned during operation.

According to the present state of technology, coal is burned influidized bed combustion systems for steam and/or electricity generation(Siemens-Energietechnik 2 (1980), Issue 7, pp. 231-235). This results intwo significant advantages. First, the combustion in the fluidized bedoccurs at relatively low temperatures so that in comparison with knowncombustions using dry or even wet ash relatively little nitrogen oxideis formed Second, the combustion in the fluidized bed places relativelylow requirements on the used coal, i.e. even a relatively roughly groundcoal with high ash content may be burned without problems. "Ash" heremeans the incombustible substances contained in the coal, such as sand,clay, salts, metal compounds, etc.

When operating a fluidized bed combustor, coal is used which is groundto a grain size specific to the fluidized bed. It is fed to thefluidized bed, i.e. in most cases via the free space located above thefluidized bed. It is then mixed homogeneously with the bedding material.This bedding material consists both of sand and ash particles which wereadded to the fluidized bed with the coal, and of migrating coalparticles.

A disadvantage here is that, in particular in the case of discontinuousash removal, the fine-particle bedding material in the fluidized bed iscontinuously reduced because of flue-dust removal with the flue gas. Therough grain content increases steadily and settles during longeroperation on the nozzle bottom of the fluidized bed. This again resultsin the obstruction of individual nozzles which feed the carrier air. Asa consequence, locally differing zones with little or no air throughputat all are generated, as well as zones with increased air throughput.This significantly impairs the temperature homogeneity of the fluidizedbed which is required for the removal of the generated heat, e.g. via aheat exchange system. In addition, temperature streaks form above theareas with increased air throughput in the free space of the fluidizedbed combustor. These are narrow, long-stretched structures which areabove the softening point of the ash. This results in slagging of thefree space, e.g. in the form of nests. These slaggings may make itnecessary to take the fluidized bed combustor out of operation. It istherefore important to balance the temperature distribution as evenly aspossible over the entire fluidized, bed so that no slag deposits mayform.

SUMMARY OF THE INVENTION

The invention has the task of providing a process of the above describedtype which makes it possible to operate a coal-based fluidized bedcombustor without problems even over a longer period. In addition, afluidized bed combustor of the above described type shall be providedwhich will make it possible to avoid the formation of slag over a longerperiod.

According to the invention the first task is solved in that part of thebedding material is removed from the bottom region of the fluidized bedcontinuously or discontinuously, is then preferably cooled, subsequentlyprocessed to a predetermined maximum grain size which is specific to thefluidized bed, and is then recycled to the fluidized bed.

According to the invention the second task is solved by a systemcharacterized by

a) a removal mechanism for the removal of part of the bedding materialwhich is located in the bottom region of the fluidized bed;

b) an ash cooler which cools the removed bedding material and which ispreferably located behind the removal mechanism;

c) a mechanism for processing the bedding material to a preestablishedmaximum grain size specific to the fluidized bed;

d) an outlet for excess bedding material; and

e) conveyor means for the transport of the processed bedding materialinto the fluidized bed.

The invented suggestion attempts to no longer adjust the optimum grainsize range of the bedding material of a fluidized bed combustor by wayof grinding the added fresh coal, but rather to achieve this through(preferably continuous) processing of the bedding material.

The bedding material may be processed here simply by way of a screeningmechanism which is adjusted to the necessary grain size range and whichseparates a fine grain and rough grain fraction in this way. In theprocess the rough grain fraction which does not pass the screeningmechanism is ground in a comminution or, specifically, a grindingmechanism and is in comminuted form recycled to the screening mechanism.In place of screening, it is also possible to use wind sizing. Thepreestablished maximum grain size may then e.g. be 3 to 4 mm.

Since according to the invention the optimum fluidized bed grain sizerange is no longer adjusted by means of the added coal, it is onlynecessary to grind the transported coal fed to the fluidized bed to arelatively rough particle size. In addition to the savings in processingand grinding costs for the coal the use of rough-grained coal has theadditional advantage that in comparison with the fine-ground coal thefree surface of the added coal is overall rather small. When the coal isadded into the free space above the fluidized bed, only a small part ofthe coal therefore evolves into gas already when it is added to thisfree space. As a result the temperature of the free space remainsrelatively low so that problems in respect to increased slag formationwill no longer occur.

A further reduction in gas evolution of the fresh fuel (coal) in thefree space above the fluidized bed may be achieved in that, according toa further development of the invention, the processed and recycledbedding material (ash) is not added into the fluidized bed separatelyfrom the fuel (coal) but is mixed with this fuel prior to this process.This increases the content of cold inert material (ash, sand, etc.)during the feeding of the coal and results during the fuel addition in atemperature reduction in the free space.

During the course of combustion the rough grain part of the coal slowlymigrates to the nozzle bottom of the fluidized bed. It is removedhere--as described--and is then processed. It is useful here that anamount of bedding material (i.e. mostly ash) is removed during each timwunit which is greater than the amount which is fed as rough grain by wayof the coal to the fluidized bed. In this way a settling of the roughgrain on the nozzle bottom of the fluidized bed may be avoided withcertainty.

The outlet for the excess bedding material may principally be located onthe nozzle bottom or in front of the screening mechanism. But it ispreferred that the outlet for excess bedding material is located in thetransport path behind the mechanism for the processing of the beddingmaterial. This is advantageous when the material removed here is placedin intermediate storage in a hopper and is then further processed. Inthis case it is already processed for further processing in thefluidized bed.

Overall, the invention makes it possible to burn coals with varyingcalorific values without problems in a fluidized bed combustor andwithout great requirements on processing and grinding.

BRIEF DESCRIPTION OF THE DRAWING

The figure is a schematic representation of a fluidized bed combustor ofthe present invention.

DETAILED DESCRIPTION OF THE DRAWING

The invention and additional versions thereof are described in thefollowing with the help of the example of a fluidized bed combustorshown schematically in the drawing.

The drawing shows a kettle 1 operated by fluidized bed combustion oncoal basis which has a fluidized bed 20 whose top level is designated byarrow 2. The carrier air 1 for the fluidized bed 20 is fed through apipe 3 into an air box 30. From there it is by way of distributornozzles 4 which are located in the nozzle bottom 9 fed into the beddingmaterial of the fluidized bed 20. In this case the bed 20 consists of amixture of coal particles and particles of coal ash, i.e. of coal andburned and unburned residues. The heat generated in the fluidized bed 20is here removed via a heat exchange system 5 which is located directlyabove the nozzle bottom 9, e.g. with the help of air or water as a heatexchange medium. The heat may be used for heating the working substancesof a gas or steam turbine which is not shown here. During operation thefluidized bed 20 has e.g. a temperature of 850° C. It is important thatduring operation the temperature is balanced as best as possible overthe entire fluidized bed 20 so that no slag which settles in the kettle1 and/or flue gas outlet may form from ash and coal particles. The fluegases g from the combustor which contain fine dust or fine ash firstpass a free space 6 which is located above the fluidized bed 20. Theyare then fed to mechanisms which separate dust and noxious substances(not shown) by means of an outlet 7, at e.g. 950° C. From there they aretransferred to a heat exchanger (not shown), e.g. in a steam generator.

Comminuted, but relatively rough-grained (roughly ground), fresh coal kis added via a conveyor path 8 above the fluidized bed level 2 into thefree space 6, e.g. with the help of a metering hopper or thrower (notshown). Alternatively, it could be added in the area of the fluidizedbed 20, e.g. through direct firing or other known methods.

In the fluidized bed 20 the direct firing with air causes the describedmixing of larger and smaller particles present. However, because ofvertical migration and force of gravity the bottom region of thefluidized bed will contain mostly rougher particles.

The bedding material a which collects in the area of the nozzle bottom 9of the fluidized bed 20 is removed by way of a removal mechanism whichin this case is constructed in the form of an outlet opening 9a locatedin the nozzle bottom 9 and by means of an ash or cellular wheel sluice10. For redundancy, several of such removal mechanisms 9a, 10 may beprovided. It is preferred that the withdrawal is continuous. The removedbedding material a, essentially ash, is then cooled, e.g. throughnatural cooling as it is left to stand, or--as in this case--throughforced cooling in an ash cooler 11.

The removed and then cooled bedding material a which includes allpossible particle or grain sizes is then transferred to a mechanism12,13 where it is processed to a preestablished maximum grain size whichis specific to the fluidized bed. This preestablished maximum qrain sizewhich may be processed further may e.g. be 4 mm. Naturally, a differentgrain size may be determined. This depends on operating parameters whichexist or are to be set. In this case the processing mechanism has ascreening mechanism 12, a comminution mechanism 13, e.g. an ash breaker,and a transport pipe 14. The screening mechanism 12 separates a finegrain fraction a', in the example with a grain size below 4 mm, and arough grain fraction A', in the example with a grain size above 4 mm.The rough grain fraction A' is transferred to the comminution mechanism13. The output of the latter is connected via pipe 14 to the input ofthe screening mechanism 12. During the course of the processing therough grain fraction A' which does not pass the screening mechanism 12is removed, comminuted in the ash breaker 13 and then transferred backto the screening mechanism 12 via the pipe 14. It is preferred that theamount of bedding material a' which is removed and processed per timeunit is greater than the amount of ash added with and contained in thecoal k in this time unit.

The bedding material a' which was processed according to the requiredgrain size is transported into the fluidized bed 20 by way of suitabletransport means. In this case a conveyor unit 15 on a conveyor path 18is used, e.g. a conveyor belt. Pneumatic transport is also possible. Thebedding material a' is preferably mixed with the fresh coal k in amixing chamber 19 in the conveyor path 8. The processed material a' isfed together with this coal k via a free space 6 into the fluidized bed20. Naturally, the material a' may also--as in the drawing--be thrownonto the fluidized bed 20 separately from the coal k.

Excess material a" is withdrawn from the system via an ash outlet or ashsluice 16 which is located behind the screening mechanism 12. This ashsluice 16 is preferably constructed adjustable; it is e.g. adjustable bymeans of a motor.

The withdrawal is performed in such a way--under consideration of ashcontent and transport of the coal k as well as the removal speed at thesluice 10--that the level 2 of the fluidized bed 20 is maintained at aconstant level. The withdrawn material a" is stored in a sand or ashhopper. It may possibly be recycled, i.e. be added to the coal k in themixing chamber 19 at a later time. The ash sluice 16 may also be locatedat a different place, e.g. in front of the screening mechanism 12 forthe removal of unprocessed bedding material a. In the shown position ithas the advantage, however, that material a" with the correct maximumgrain size is already provided for further processing at a later time.

The described processing ensures that the required grain size range 0 upto e.g. 4 mm for the bedding material is consistently approximatelymaintained within the fluidized bed 20. No major requirements exist forprocessing and grinding of the fresh coal k itself. A relativelyrough-grained coal k may be used which results in the conservation ofprocessing and grinding costs.

We claim:
 1. Process for operating a coal-based fluidized bed combustorin which comminuted coal (k) is fed to a fluidized bed (20) and isburned together with the bedding material, characterized in that part ofthe bedding material from the bottom region of the fluidized bed (20) iswithdrawn continuously or discontinuously, is then preferably cooled,then processed to a preestablished maximum grain size specific to thefluidized bed, and is then recycled back to the fluidized bed (20),further characterized in that the recycled bedding material (a') ismixed with the coal (k) prior to its addition to the fluidized bed (20).2. Process according to claim 1, characterized in that the amount ofbedding material (a') withdrawn and processed per time unit is greaterthan the amount of ash added with and contained in the coal k. 3.Process according to claim 2, characterized in that the withdrawnbedding material (a) is screened during processing so that a fine grainfraction (a') and a rough grain fraction (A') is generated whereby therough grain section (A') has a grain size which is larger than thepreestablished maximum grain size specific to the fluidized bed, andthat the rough grain fraction (A') is comminuted and screened togetherwith additional withdrawn bedding material (a).
 4. Fluidized bedcombustor with a conveyor path (8) for the transport of the coal (k) anda nozzle bottom (9) on which a fluidized bed is located which consistsduring operation of bedding material, characterized bya) a removalmechanism (9a , 10) for the removal of part of the bedding material (a)which is located in the bottom region of the fluidized bed (20); b) anash cooler (11) which cools the removed bedding material (a) and whichis preferably located behind the removal mechanism (9a, 10); c) amechanism (12, 13) for processing the bedding material (a) to apreestablished maximum grain size specific to the fluidized bed; d) anoutlet (16) for excess bedding material (a or a'); and e) conveyor means(15,18) for the transport of the processed bedding material (a') intothe fluidized bed (20),further characterized in that the mechanism(12,13) for the processing of the bedding material (a) includes ascreening mechanism (12) which separates a fine grain fraction (a') anda rough grain fraction (A'), further characterized in that the screeningmechanism (12) has a correlating comminuting mechanism (13) to which therough grain fraction (A') separated by the screening mechanism (12) maybe fed and whose output is connected to the input of the screeningmechanism (12).
 5. Fluidized bed combustor according to claim 4,characterized in that the conveyance means (15,18) include a conveyorbelt (18).
 6. Fluidized bed combustor according to claim 4,characterized in that a mixing chamber (19) is provided where theprocessed bedding material (a') may be mixed with the coal (k). 7.Fluidized bed combustor according to claim 4, characterized in that theoutlet (16) for excess bedding material (a or a') is located in thetransport path behind the mechanism (12,13) for the processing of thebedding material (a).
 8. Fluidized bed combustor according to claim 4,characterized in that the removal mechanism (9a,10) includes at leastone cutlet opening (9a) which is located in a nozzle bottom (9). 9.Fluidized bed combustor according to claim 8, characterized in that aheat exchange system (5) is located above the nozzle bottom (9). 10.Fluidized bed combustor according to claim 4, characterized in that thepreestablished maximum grain size specific to the fluidized bed is ca. 3to 4 mm.